The present invention relates to a process for overcoating granules with overcoating material, especially for covering coated fuel particles with binder-containing graphite powder for high-temperature reactors. In carrying out the process, there is employed an apparatus comprising a granulator and pipes for adding overcoating material and liquid in measured portions.
Granulation as a process has been introduced in many areas of industry. Processes and apparatus therefor are known in numerous versions. The production of granules that do not turn into dust and are readily free-flowing is an important field of application. Another field is the overcoating of granular materials with specified layers. Thus, in the pharmaceutical industry, layers containing active ingredient are applied on granular backing materials. Also, when making nuclear fuel elements for high-temperature reactors, overcoating layers are applied on particles by granulation. In this case, the nuclear fuel particles coated with pyrocarbon and, if necessary, also with silicon carbide are provided before further processing with an additional overcoating layer of binder-containing graphite powder. These thusly overcoated fuel particles are then pressed under high pressure to form a graphitic matrix of the same material as the overcoating layer. The object of the pyrocarbon layers and, in some cases, of the silicon carbide layer is to retain in the fuel element the solid and gaseous fission products forming during reactor operation.
Therefore, these layers must not be damaged during the fabrication of the fuel elements, especially during the forming step by pressing. Even very high isostatic compressive loads must not lead to damage to the layers. Nevertheless, rupture of the layers occurs if coated particles come into contact with each other while being pressed into the matrix and local pressure peaks develop due to these point contacts.
The overcoating of the coated fuel particles with binder-containing graphite powder is for the purpose of preventing such point contacts by means of a spacing or blocking function. For this reason, the requirements for the quality of the overcoating layer, especially with respect to uniformity, is very high.
A disadvantage, especially with respect to quality assurance, is that the granulation process is generally very dependent on the skill of the operating personnel. The starting phase of the process in particular requires sure considerable skill and experience on the part of the operator. He proportions the powder and adds the liquid according to the flow behaviour of the granules and must correct the flow behavior, often even manually.
It has also proved very disadvantageous when applying granulation to the field of fuel element fabrication for high-temperature reactors, in that layer thicknesses of 200 micron and higher, as are necessary for particles with an additional silicon carbide layer, can be applied with the necessary uniformity only if the overcoating process is interrupted several times and the particles are separated with the desired layer thickness. If this separation does not occur, a very wide range layer-thickness distribution is achieved which is not acceptable for the purpose at hand.